The UN says it will need a record $16bn (£10.5bn; 13.3bn euros) to fund its humanitarian operations next year, with almost half the total going to help victims of the Syrian conflict.
It says the money will provide aid for more than 57 million of the most vulnerable people around the world.
The UN humanitarian chief said the level of need was "unprecedented".
The request comes as aid agencies warn they are running out of cash to fund this year's operations in Syria.
Last week the World Food Programme announced it would have to cut food rations to Syrian refugees.
The UN is requesting $2.8bn to help those displaced by the conflict inside Syria.
It is seeking another $4.4bn to help more than 3,250,000 Syrian refugees registered in neighbouring countries.
"The rising scale of need is outpacing our capacity to respond," said UN Undersecretary-General for Humanitarian Affairs Valerie Amos. "The crises in Central African Republic, Iraq, South Sudan and Syria will remain top humanitarian priorities next year."
Ms Amos said those conflicts accounted for more than 70% of the funding being sought.
Analysis: Lyse Doucet, Chief international correspondent
This year's record appeal underlines again that what is needed most of all to address a deepening humanitarian crisis are political solutions. The lion's share of the money - about 80% in total - will go towards helping vulnerable people struggling to survive in areas of conflict.
Almost all of them live in four countries - Syria, Iraq, South Sudan, and the Central African Republic. And in all of these conflicts there is no political solution in sight.
As Syria's punishing war drags on, the UN envoy tasked with finding a way out is now focusing on a "freeze" in hostilities to allow for the distribution of badly needed aid. But in many conflict zones, appeals to warring parties often fall on deaf ears. And while donors are giving more money, it's simply not enough to match growing needs.
For the last few years the UN spoke of a record request for aid, mainly for Syria which is now described as the biggest humanitarian test of our time. Next year is likely to be no different.
'Not business as usual'
Other major crises covered by the appeal include Afghanistan, DR Congo, Myanmar, Palestinian territories, Somalia, Sudan, Ukraine and Yemen.
However, the UN said it did not include nine countries in Africa's Sahel region, which will be addressed in a separate request in February.
UN High Commissioner for Refugees Antonio Guterres said: "This is not business as usual in the humanitarian world. Today's needs are at unprecedented levels, and without more support there simply is no way to respond to the humanitarian situations we're seeing."
The BBC's Imogen Foulkes in Geneva says relief never happens overnight - aid agencies need to plan, but to do that they need cash.
Food and medical supplies for refugees have to be purchased in advance, and field hospitals have to be delivered and built.